1. Field of the Invention
The present invention relates to an optical system which is appropriate for an optical apparatus such as a silver-halide film camera, a digital still camera, a video camera, a telescope, a binoculars, a projector, or a copying machine.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2007-333790 discuses a Gauss lens (optical system) as a standard photographic lens having a photographic angle of view of about 45 degrees used for a single-lens reflex camera. Typically, a Gauss lens consists of two lenses; in its most basic form, a positive meniscus lens on the object side and a negative meniscus lens on the image side. The power of the positive lens generally predominates, but the negative lens corrects for chromatic aberration. The Gauss lens has features of having relatively long back focus, a wide angle of view, and a large diameter ratio and capable of easily obtaining high optical performance. In addition, Japanese Patent Application Laid-Open No. 05-142474 discuses a Gaussian photographic lens, in which focusing is performed by changing a relative relationship between lens units so as to reduce variation in aberration due to a change in object distance.
In a large-aperture-ratio Gauss lens, in order to achieve high optical performance in a range from on the axis up to the margin of the image plane, a lanthanum flint glass containing lanthanum is mainly used as a material of a positive lens, and a titanium flint glass containing titanium is mainly used for a negative lens.
However, if such a material is used, a secondary spectrum of axial chromatic aberration is increased, so that it is difficult to correct the secondary spectrum of axial chromatic aberration. Therefore, in a large-aperture-ratio Gauss lens, it is important to reduce color flare caused by the secondary spectrum of axial chromatic aberration.
Therefore, with respect to the Gaussian lens or a modified Gauss lens as a modification of the Gaussian lens, in order to obtain high optical performance by reducing the secondary spectrum of axial chromatic aberration, it is important to appropriately select a material of each lens. For example, it is important to use the lenses of which a refractive index, Abbe number, partial dispersion ratio (relative partial dispersion), and the like of a material are appropriately selected.